<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">发送验证码</button>
    <script>
        var btn = document.getElementById('btn')
        var num = 5;
        btn.onclick = function(){
          var timer = setInterval(function(){
            if(num>0){
                btn.innerHTML = num;
                btn.disabled = true;
                num--;
            }
            else{
                clearInterval(timer);
                btn.innerHTML = '发送验证码';
                btn.disabled = false;
                num =5 
            }
          },1000)
        }
    </script>
</body>
</html>